Despite standard CPR, the vast majority of patients in cardiac arrest never survive to hospital discharge. Simple and effective methods for improving the efficacy of CPR are needed. The inspiratory threshold valve (ITV(tm)) is a new device designed to increase negative intrathoracic pressure and venous blood returns to the heart, thereby improving vital organ blood flow during CPR. It is attached to a face mask or breathing tube during CPR. In Phase I, investigators established proof of concept by demonstrating that use of the ITV TM significantly increased 24-hour survival and neurological function after cardiac arrest in pigs. Phase I results now support a prospective, randomized, blinded, out-of-hospital clinical trial by the applicants of this grant proposal, approved for implementation by the FDA (IDE: G9801 25/S3). In Phase II, investigators will: 1) compare intensive care unit admission rates in patients with an out-of-hospital cardiac arrest receiving standard CPR with either an active or a sham ITV TM; 2) compare peak ETCO2 values during CPR between the two groups and retrospectively correlate values with outcome; and 3) compare key hemodynamic parameters in a subset of patients. If Phase II is successful, the ITV TM valve has the potential to resuscitate approximately 50,000 more patients per year in the United States alone. The potential market size for this small, inexpensive, lightweight valve is substantial. It can be easily added to all existing Basic and Advanced Life Support resuscitation equipment.